Fiber-reinforced concrete cask, supporting frame for molding thereof and process for producing the concrete cask

ABSTRACT

The object of this invention is to provide a fiber-reinforced concrete cask that ensures easy working, enables reducing working cost, excels in strength, durability and heat resistance and enables minimizing cracking; a process for fabrication of the same; and a supporting frame for molding the concrete cask. In particular, concrete cask ( 10 ) formed through injecting and solidification of concrete ( 11 ) is characterized in that sheets of reinforcement fibers having a thermal expansion coefficient equal to or lower than that of concrete ( 11 ) are provided on at least the outer circumferential surface and the inner circumferential surface of the concrete cask ( 10 ) and that the inner circumferential surface of outer sheet ( 21 ) and the outer circumferential surface of inner sheet ( 22 ) are connected with each other by strings of reinforcement fibers ( 23 ). Preferably, carbon fibers are used as the reinforcement fibers.

FIELD OF TECHNOLOGY

The present invention relates to a fiber reinforced concrete cask suchis used for the transport and storage of radioactive materials, as wellas a support frame for molding thereof, and a process for fabrication ofthe concrete cask.

BACKGROUND TECHNOLOGY

When storing or transporting radioactive substances, generated bynuclear power plants such as spent fuel having a high level ofradioactivity and decay heat, the container used to hold this materialmust have a high radioactivity shielding capability, high sealperformance, and have adequate cooling capabilities and structuralstrength. In general, concrete reinforced with steel rods or sheets hasbeen used to fabricate such containers, but problems remain in thecurrent implementations. One of the problems is the difference in thecoefficients of thermal expansion between the concrete and the steelreinforcing materials.

Internally or externally reinforcing concrete using steel materialsimproves the strength of the container, but since the coefficient ofthermal expansion of the steel materials is greater than that of theconcrete, if the materials inside the container emit heat, cracks in theconcrete could be generated to damage the container. Further, since theheat conductivity of concrete is lower than that of the metal, theadditional problem being difficult to dispel heat generated inside thecontainer to the outside exacerbates the above cited differences intheir coefficients of thermal expansion even more to increase crackformation.

At this point, JP2000-162384A discloses a concrete cask which preventsthe concrete cask container itself from reaching high temperatures.

As is shown in FIG. 4, the concrete cask 51 is comprised of concrete 55formed to a cylindrical shape with a bottom, and an inner metal cylinder56 on the inside circumference of container unit 53. Then, canister 52is inserted into the container and the top opening is sealed by lid 54.A space 57 for the circulation of cooling air is disposed between theoutside surface of canister 52 and container unit 53 and cooling airsupply passages 58 and cooling air exhaust passages 59 are formed toconnect thereto.

Thus, the structure enables cooling air to exhaust the heat from theinside of the container unit to the outside to thereby improve thedurability and heat resistance of the container.

It is further disclosed to use a metal such as stainless steel, whichhas a coefficient of thermal expansion approximately equivalent to thatof the concrete, to form the inner metal cylinder 56 as a reinforcingmaterial for the concrete cask, as a means to minimize any damage to thecask and help it maintain its strength.

Further, JP2000-265435A discloses the use of polyethylene or other fibersheets as a support frame instead of using steel as a reinforcingmaterial to thereby simplify fabrication and reduce fabrication costsfor concrete structures. According to this cited invention, a jacketwould be formed from an outer sheet and inner sheet with a spacedisposed between them, and then the jacket would be immersed into thesea so as to introduce sea water into the space in the jacket, whichwould then be filled with concrete, which would displace the water, andbe subsequently allowed to solidify to complete the structure.

However, just as is the case with the above cited JP2000-162384A, bysimply forming passages for the flow of cooling air, when used tocontain a high temperature heat emitting material with a high heatoutput, differences in the coefficients of thermal expansion of thematerials could not be absorbed, and cracking would be inevitable.Further, not only does the use of stainless steel materials as disclosedmake the fabrication more difficult, but it also entails much highermaterial costs.

On the other hand, while concrete structures according to the foregoingPatent Publication 2000-265435 would be suitable for holding lowtemperature materials, the use of polyethylene, or other fiber sheetsfor the support frame poses problems in the areas of both strength andheat resistance.

DISCLOSURE OF THE INVENTION

The present invention was developed after reflecting upon the problemsassociated with the prior art. The object of this invention is toprovide a fiber-reinforced concrete cask that ensures easy working,enables reducing working cost, excels in strength, durability and heatresistance and enables minimizing cracking; a process for fabrication ofthe same; and a support frame for molding the concrete cask.

To resolve the above problems the present invention is characterized inthat the fiber reinforced concrete cask is formed by injecting andsolidifying concrete wherein reinforcement fiber sheets are disposed atleast on an outside circumference surface of said cask, and saidreinforcement fiber sheets have a coefficient of thermal expansionequivalent to or less than that of the concrete.

In this case, the reinforcement fiber sheets are preferably disposed onboth the outside circumference surface and the inside circumferencesurface of said concrete cask, and said reinforcement fiber sheets onsaid outside and inside circumference surfaces are connected withstrings.

Further, the reinforcement fiber sheets are preferably carbon fibers.

According to the invention disclosed above, it is possible to fabricateconcrete casks having superior durability and heat resistance withoutthe cracking or dissociation from the reinforcement material seen in theprior art that was caused by expansion or pulling away from the concreteof the steel reinforcement materials or frames utilized in the casksthat were caused by heat generation from the cask's contents.

Additionally, the present invention is characterized in that the fiberreinforced concrete cask is formed by injecting concrete into andsolidifying within a cylindrical bag support frame formed fromreinforcement fiber sheets that have a coefficient of thermal expansionequivalent to or less than that of the concrete. What is meant by theaforementioned “cylindrical bag,” are bag-shaped cylindrical structuresthat include hollow cylindrical shapes, hollow cylindrical shapes with abottom (a cylindrical container), and structures where the base plateincludes true cylindrical forms.

Further, through the use of carbon fibers that have a negativecoefficient of thermal expansion as the foregoing reinforcement fibers,such carbon fibers contract in response to rising temperatures from theheat generated inside the cask to exert compression force upon theconcrete, which is weak with respect to tensile forces, but strong withrespect to compression forces, thereby making it possible todramatically improve the strength of the concrete.

It is necessary that the foregoing reinforcement fibers are strongenough to withstand the injection of the concrete, and that the fibershave sufficiently high heat resistance to withstand the heat fromheat-emitting materials. It is further preferable that theaforementioned strings are formed from reinforcement fibers such ascarbon fibers.

Further still, the present invention is characterized in that thesupport frame is made from reinforcement fiber sheets having acoefficient of thermal expansion that is equivalent to or less than thatof the concrete.

Also, it is further preferable in this invention that the support framehas a double walled structure made from said reinforcement fiber sheetscomprising an outer sheet and an inner sheet joined together, and saidouter sheet and inner sheet are joined by strings, and the support frameis sewn together into a cylindrical bag shape, and made fromreinforcement fiber sheets.

As previously stated, what is meant by “cylindrical bag” shaped includesbag-shaped cylindrical structures that include hollow cylindricalshapes, hollow cylindrical shapes with a bottom (a cylindricalcontainer), and structures where the base plate includes truecylindrical forms.

According to this invention, it is possible to form a support frame forthe concrete cask that will deliver the aforementioned operationaleffects of this invention. It is further preferable that a concreteinjecting input opening is located in the lower part of the foregoingsupport frame according to the present invention.

The preferred method for the fabrication of a concrete cask according tothe present invention is characterized in that it includes theprocesses: forming a support frame for injection of the concrete, usingreinforcement fiber sheets having a coefficient of thermal expansionequivalent to or less than that of the concrete, and injecting theconcrete into said support frame. The process for forming the foregoingsupport frame includes preferably the joining of the outer sheet andinner sheet of reinforcement fibers comprising said support frame withstrings.

By doing so, although tensile forces remain in the sheets of the supportframe from the pressure exerted upon them during the injection of theconcrete, since the concrete exhibits no resiliency after it has cured,said sheets then contract, which puts a compressive pre-stress on theconcrete from the outside. This makes it possible to effectively use aconcrete structure which is characteristically weak to the tensile forcebut strong to the compression force.

It is further preferable following the process to form the foregoingsupport frame, to include a process for filling said formed supportframe with a fluid that will maintain a shape of said support frame, anda process for injecting the concrete from a bottom of said support framein said concrete injecting process to replace said fluid previouslyfilled into said support frame to hold said shape, with the concrete.

The fluid used to maintain the shape of the foregoing support frameshould be, for ease of operations, one with a lower specific gravitythan the concrete and easy-care such as water, air, etc.

According to this invention, by pre-filling the support frame with afluid to hold its shape and by replacing it with concrete, it ispossible to fabricate the concrete cask to accurate dimensions withoutthe necessity of taking the trouble to prepare a mold frame such as asteel frame.

To wit, the present invention not only makes it possible to easefabrication and lower fabrication costs, but the invention can providethe concrete cask which additionally makes it possible to improve thestrength, durability and heat resistance and to minimize any crackgeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an embodiment of a fiber reinforcedconcrete cask according to the present invention.

FIG. 2 shows sectional views: (a) a sectional view along line A-A ofFIG. 1, and (b) a sectional view along line B-B of FIG. 2( a).

FIG. 3 is a diagram showing the fabrication process for this embodimentof a fiber reinforced concrete cask according to the present invention.

FIG. 4 shows a perspective outer view of a concrete cask according tothe prior art.

A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Embodiments of the present invention will be described below with thereference of the attached drawings. In these embodiments, unlessotherwise stated, any specific mention of such details as thedimensions, materials, or relative positioning of any of the componentparts should not be construed as to limit the scope of this invention;they are merely included for purposes of explanation.

FIG. 1 shows a perspective view of an embodiment of a fiber reinforcedconcrete cask according to the present invention; FIG. 2 shows sectionalviews: (a) a sectional view along line A-A of FIG. 1, and (b) asectional view along line B-B of FIG. 2( a); and FIG. 3 is a diagramshowing the fabrication process for this embodiment of a fiberreinforced concrete cask according to the present invention.

This embodiment of a fiber reinforced concrete cask would be used as acontainer to store or transport radioactive material generated in anuclear power plant such as spent fuel, recycled fuel, etc.

In FIG. 1 and FIGS. 2( a) and (b), fiber reinforced concrete cask 10according to the present embodiment is comprised of support frame 20,formed by sewing together an outer sheet 21 and a smaller diameter innersheet 22 into a cylindrical shaped bag having a bottom, and concrete 11contained in the bag. Although not shown in the figures, its structureis such that it can contain a canister holding radioactive materials.Further, in order to facilitate shape retention of the foregoing supportframe 20, a plurality of strings 23 join the inside circumference of theforegoing outer sheet 21 with the outside circumference of inner sheet22.

Reinforcement fibers are used in the foregoing outer sheet 21, innersheet 22 and strings 23. Said reinforcement fibers, at least for outersheet 21, must have a coefficient of thermal expansion equivalent to orless than that of thermal expansion of the concrete. To wit, if theconcrete used to fill support frame 20 has a thermal expansioncoefficient ranging from about 0.5 to 1.5×10⁻⁵/° C., the reinforcementfibers used for support frame 20 must have a coefficient of thermalexpansion equal to or less than approximately 1.5×10⁻⁵/° C. It ispreferable that the reinforcement fibers can be high strength, heatresistant fibers having a negative coefficient of thermal expansion,such as carbon fibers. It is, of course, preferable to use high strengthfibers with the aforementioned properties for the foregoing inner sheet22 and the strings 23 as well.

Further, the foregoing support frame 20 should have an injection port 12in the lower part thereof as well as a fluid outlet port 13 in the upperpart thereof. The foregoing injection port 12 should be of a structurewhich can be connected to the supply hose of the concrete to be injectedinto the support frame, and when the concrete is injected, the supplyhose and the foregoing injection port 12 are sealed off with a hoseclamp.

On the other hand, the fluid outlet port 13 should preferably beequipped with a cock or other type of valve to facilitate the belowdescribed expelling of the shape retention fluid, and to make itpossible to seal off the inside of the foregoing support frame with theforgoing valve.

Further, a plurality of strings 23, which help the foregoing supportframe 20 hold its shape, should be installed in the circumferential andin the height directions of said support frame 20; the number installedshould be the number required for the support frame to retain its shapewhen it is filled with concrete.

Further, flange 15 is fabricated in the top inside circumference of theforegoing concrete cask 10, which can accommodate the insertion of lidmember 14. Said flange 15 is preferably formed as a projection on theinside circumference of the foregoing inner sheet 22 by filling it withconcrete 11, and lid member 14 can be formed by filling a bag-shaped lidframe made from reinforcement fibers with concrete 11 in a mannersimilar to that described above for support frame 20.

Also, in the present embodiment, concrete cask 10 is a unitized hollowcylinder having a bottom, but the body of the hollow cylindrical shapeand the bottom of the cylinder and the lid also may be comprised of 3respective blocks, or an even greater number of blocks, that areindividually fabricated and joined together to form a unitized concretecask.

Further, it is also preferable that there are air supply ports andexhaust ports established in the sides of concrete cask 10 toaccommodate the supply and exhausting of the air that is circulated inthe space between the inside circumferential wall of said concrete caskand the outside of the canister contained within concrete cask 10.

Using the above described structure, even if heat generated inside theforegoing canister causes concrete 11 to expand, the support frame 20,which has a coefficient of thermal expansion that is less than that ofsaid concrete, protects the concrete and generates the special effect ofincreasing its compression strength as a tradeoff for weakened tensilestrength.

Further, by using carbon fibers as the foregoing reinforcement fibers,it is possible to provide concrete casks 10 having excellent strengthand heat resistance.

Next, the fabrication method for the fiber reinforced concrete cask ofthe present embodiment will be described with reference to FIG. 3.

First, as shown in FIG. 3( a), the outer sheet and inner sheet of carbonor other reinforcement fibers are sewn together. The reinforcementfibers, as explained above, must have a coefficient of thermal expansionthat is equivalent to or less than that of concrete, and additionally,they must be strong and resistant to heat. Sheets of reinforced fiberswoven to the required size, or rectangular shaped sheets blocks of theappropriate size are sewn together to create cylindrical shaped sheets.

The diameter of the inner sheet is smaller than the diameter of theouter sheet by an amount equivalent to the desired thickness of thecask. It would also be possible to bind with adhesives or fuse thereinforcement fiber sheets instead of sewing them together. Also, theoutside circumferential surface of inner sheet 22 is joined with theinside circumferential surface of outer sheet 21 by a plurality ofstrings 23, also made from reinforcement fibers, and in addition, abottom made from circle-shaped reinforcement fiber sheet is sewntogether with the sheets, and a ring-shaped reinforcement fiber sheet issewn to the top of the sheets for fabricating the bag-shaped supportframe.

Then, as shown in FIG. 3( b), a support frame shape retention fluid 16is injected through injection port 12 in the bottom of the foregoingsupport frame 20. To facilitate operations, said fluid should be onewhich has a lower specific gravity than the concrete such as air orwater and also easy-care, and it should be one which easily separatesout from the concrete. Then, as shown in FIG. 3( c), with the supportframe 20 filled with said fluid 16, stays 26 are attached to facilitateits shape retention as well as to prevent its falling over.

Next, as shown in FIG. 3( d), a concrete supply pump is connected to theforegoing injection port 12 and concrete 11 is injected. At the sametime, the valve at the fluid outlet port 13 installed on the top ofsupport frame 20 is opened to allow the fluid 16 to be expelled. Thus,as concrete 11 is inserted from the bottom, the lower specific gravityfluid is output from the top, until concrete has replaced all of fluid16 inside support frame 20 as shown in FIG. 3( e).

When the concrete injecting into said support frame 20 has beencompleted, the injection of concrete is halted and it is allowed to curefor the required period of time. Thus, with the solidification ofconcrete 11 inside support frame 20, the fabrication of the concretecask is completed.

The use of this method makes it possible to simplify fabrication andreduce fabrication costs in producing concrete casks that provideexcellent heat resistance and strength.

When using water as the fluid 16 for retaining the shape of the supportframe, it is preferable to use a type of concrete materials for concrete11 that exhibits very little separation in aqueous environments.

EFFECTS OF THE INVENTION

According to this invention disclosed above, it is possible to fabricateconcrete casks having superior durability and heat resistance withoutthe cracking or dissociation from the reinforcement material seen in theprior art that was caused by expansion or pulling away from the concreteof the steel reinforcement materials or frames utilized in the casksthat were caused by heat generation from the cask's contents.

Further, through the use of carbon fibers that have a negativecoefficient of thermal expansion as the foregoing reinforcement fibers,such carbon fibers contract in response to rising temperatures from theheat generated inside the cask to exert compression force upon theconcrete, which is weak with respect to tensile forces, but strong withrespect to compression forces, thereby making it possible todramatically improve the strength of the concrete.

By doing so, although tensile forces remain in the sheets of the supportframe from the pressure exerted upon them during the injection of theconcrete, since the concrete exhibits no resiliency after it has cured,said sheets then contract, which puts a compressive pre-stress on theconcrete from the outside. This makes it possible to effectively use aconcrete structure which is characteristically weak to the tensile forcebut strong to the compression force.

To wit, the present invention not only makes it possible to easefabrication and lower fabrication costs, but the invention can providethe concrete cask which additionally makes it possible to improve thestrength, durability and heat resistance and to minimize any crackgeneration.

1. A fiber reinforced concrete cask formed by injecting into andsolidifying concrete within a support frame, wherein reinforcement fibersheets are disposed at least on an outside circumference surface of saidcask, said reinforcement fiber sheets have a coefficient of thermalexpansion less than a coefficient of thermal expansion of the concrete,and said support frame is sewn together into a cylindrical bag shape andmade from reinforcement fiber sheets.
 2. The fiber reinforced concretecask according to claim 1, wherein said reinforcement fiber sheets aredisposed on both the outside circumference surface and the insidecircumference surface of said concrete cask, and said reinforcementfiber sheets on said outside and inside circumference surfaces areconnected with strings.
 3. The fiber reinforced concrete cask accordingto claim 1, wherein said reinforcement fiber sheets are carbon fibersand sewn together into the cylindrical bag shape to form bag-shapedcylindrical structures, and said bag-shaped cylindrical structuresinclude hollow cylindrical shapes, hollow cylindrical shapes with abottom, and structures where a base plate includes cylindrical forms. 4.The fiber reinforced concrete cask according to claim 1, wherein saidsupport frame has an injection port in a lower part of said supportframe.
 5. A support frame for forming a concrete cask, wherein saidsupport frame is made from reinforcement fiber sheets having acoefficient of thermal expansion that is less than a coefficient ofthermal expansion of concrete used to form the concrete cask, and saidsupport frame is sewn together into a cylindrical bag shape and madefrom the reinforcement fiber sheets.
 6. The support frame for formingthe concrete cask according to claim 5, wherein said support frame has adouble walled structure made from said reinforcement fiber sheetscomprising an outside sheet and an inside sheet joined together, andsaid outside sheet and inside sheet are joined by strings.
 7. Thesupport frame for forming the concrete cask according to claim 5,wherein said support frame has an injection port in a lower part of saidsupport frame.
 8. A method of fabricating a concrete cask, comprising:forming a support frame for injection of concrete, using reinforcementfiber sheets having a coefficient of thermal expansion less than acoefficient of thermal expansion of the concrete, and injecting theconcrete into said support frame.
 9. The method of fabricating theconcrete cask according to claim 8, wherein the forming the supportframe includes using reinforcement fiber sheets that include an outsidesheet and an inside sheet joined together by reinforcement fiberstrings.
 10. The method for the fabrication of fabricating the concretecask according to claim 8, further comprising filling said formedsupport frame with a fluid that will maintain a shape of said supportframe, and wherein the injecting the concrete is performed after thefilling the formed support frame with the fluid and includes injectingthe concrete from a bottom of said support frame to replace said fluid,which is pre-filled into said support frame to hold said shape, with theconcrete.
 11. The method of fabricating the concrete cask according toclaim 8, wherein said injecting the concrete is performed so thattensile forces remain in said reinforcement fiber sheets of said supportframe from pressure exerted upon said sheets during said injecting theconcrete.